Surface treatment for photovoltaic device

ABSTRACT

The present invention relates generally to semiconductor devices and more particularly, to a structure and method of forming a photovoltaic cell using a surface treatment to improve device performance. Embodiments of the present invention may improve open circuit voltage, fill factor, and energy conversion efficiency by performing a surface treatment on an upper surface of an absorber layer. The surface treatment may improve device performance by permitting a more cohesive interface between the upper surface of the absorber layer and a lower surface of a passivation layer. The more cohesive interface may allow carriers to move from one layer to another with less resistance, and thus, increase device performance.

BACKGROUND

The present invention relates generally to semiconductor devices andmore particularly, to a structure and method of forming a photovoltaiccell using a surface treatment to improve device performance.

Electricity generated from photovoltaic cells may not be in grid paritywith other electric generating units in many parts of the United States.To achieve widespread grid parity, substantial reductions in cost andimprovements in performance may need to be achieved. Cost reductions maybe achieved through innovative photovoltaic cell fabrication techniques.Performance improvements may be achieved by improving efficiency, whichmay be accomplished by improving short circuit current, fill factor, andopen circuit voltage.

In a photovoltaic cell, surface level pinning may exist at a junctionbetween layers due to a high surface state density. Surface levelpinning may limit open circuit voltage and reduce fill factor in aphotovoltaic cell. Overcoming strong surface Fermi level pinning mayincrease open circuit voltage and fill factor in a photovoltaic device,thus increasing device efficiency. However, overcoming strong surfaceFermi level pinning may be challenging.

SUMMARY

According to an embodiment, a method is disclosed. The method mayinclude: performing a hydrogen fluoride cleaning process on the surfaceof the absorber layer; performing an ammonium sulfide passivationprocess on the surface of the absorber layer; and performing a plasmasurface treatment on the surface of the absorber layer, exposing thesurface of the absorber layer to a plasma, wherein the plasma transformsinto a solid state forming a thin solid layer on the surface of theabsorber layer.

According to an embodiment, another method is disclosed. The method mayinclude: forming a back surface layer on an upper surface of asubstrate; forming an absorber layer on an upper surface of the backsurface layer; performing a surface treatment on an upper surface of theabsorber layer, the surface treatment comprising a hydrogen fluoridecleaning process, an ammonium sulfide passivation process, and a plasmasurface treatment, wherein the plasma surface treatment exposes theupper surface of the absorber layer to a plasma; forming a passivationlayer on the upper surface of the absorber layer; forming an emitterlayer on an upper surface of the passivation layer; forming atransparent electrode layer on an upper surface of the emitter layer;forming a front contact layer on an upper surface of the transparentelectrode layer; and forming a back contact layer on a bottom surface ofthe substrate.

According to an embodiment, a structure is disclosed. The structure mayinclude: a back surface layer on an upper surface of a substrate; anabsorber layer on an upper surface of the back surface layer; a sulfurmonolayer on an upper surface of the absorber layer; a thin solid layeron an upper surface of the absorber layer; a passivation layer on theupper surface of the absorber layer; an emitter layer on the uppersurface of the passivation layer; a transparent electrode layer on anupper surface of the emitter layer; a front contact layer on an uppersurface of the transparent electrode layer; and a back contact layer ona bottom surface of the substrate.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The following detailed description, given by way of example and notintended to limit the invention solely thereto, will best be appreciatedin conjunction with the accompanying drawings, in which not allstructures may be shown.

FIG. 1 is a cross section view illustrating a structure comprising asubstrate, according to an embodiment of the present invention.

FIG. 2 is a cross section view illustrating forming a back surface fieldlayer, according to an embodiment of the present invention.

FIG. 3 is a cross section view illustrating forming an absorber layer,according to an embodiment of the present invention.

FIG. 4 is a cross section view illustrating performing a surfacetreatment on an upper surface of the absorber layer, according to anembodiment of the present invention.

FIG. 5 is a cross section view illustrating forming a passivation layer,according to an embodiment of the present invention.

FIG. 6 is a cross section view illustrating forming a emitter layer,according to an embodiment of the present invention.

FIG. 7 is a cross section view illustrating forming a transparentelectrode layer, according to an embodiment of the present invention.

FIG. 8 is a cross section view illustrating forming a front contactlayer, according to an embodiment of the present invention.

FIG. 9 is a cross section view illustrating forming a back contactlayer, according to an embodiment of the present invention.

The drawings are not necessarily to scale. The drawings are merelyschematic representations, not intended to portray specific parametersof the invention. The drawings are intended to depict only typicalembodiments of the invention. In the drawings, like numbering representslike elements.

DETAILED DESCRIPTION

Detailed embodiments of the claimed structures and methods are disclosedherein; however, it can be understood that the disclosed embodiments aremerely illustrative of the claimed structures and methods that may beembodied in various forms. This invention may, however, be embodied inmany different forms and should not be construed as limited to theexemplary embodiments set forth herein. Rather, these exemplaryembodiments are provided so that this disclosure will be thorough andcomplete and will fully convey the scope of this invention to thoseskilled in the art.

For purposes of the description hereinafter, the terms “upper”, “lower”,“right”, “left”, “vertical”, “horizontal”, “top”, “bottom”, andderivatives thereof shall relate to the disclosed structures andmethods, as oriented in the drawing figures. It will be understood thatwhen an element such as a layer, region, or substrate is referred to asbeing “on”, “over”, “beneath”, “below”, or “under” another element, itmay be present on or below the other element or intervening elements mayalso be present. In contrast, when an element is referred to as being“directly on”, “directly over”, “directly beneath”, “directly below”, or“directly contacting” another element, there may be no interveningelements present. Furthermore, the terminology used herein is for thepurpose of describing particular embodiments only and is not intended tobe limiting of the invention. As used herein, the singular forms “a,”“an,” and “the” are intended to include the plural forms as well, unlessthe context clearly indicates otherwise.

For the purposes of the description hereinafter, the terms “solar cell”,“photovoltaic cell”, and derivatives thereof shall relate generally to adevice that converts light energy to electrical energy by thephotovoltaic effect. Solar cells may include a semiconductor materialthat absorbs photons from light. When photons are absorbed, valance bandelectrons present in the semiconductor material may become excited, jumpto the conduction band, and become free. The free electrons may thendiffuse through the semiconductor material. Some of the free electronsmay reach a junction where they are accelerated into a differentmaterial, typically a metal contact, by a built-in potential. Thismovement generates an electromotive force, thus converting some of thelight energy into electric energy.

For the purposes of the description hereinafter, the term “back” andderivatives thereof shall relate generally to an element such as alayer, region, or substrate near a back contact layer. In contrast, theterm “front” and derivatives thereof shall relate generally to anelement, region, or substrate near a front contact layer.

In the interest of not obscuring the presentation of embodiments of thepresent invention, in the following detailed description, someprocessing steps or operations that are known in the art may have beencombined together for presentation and for illustration purposes and insome instances may have not been described in detail. In otherinstances, some processing steps or operations that are known in the artmay not be described at all. It should be understood that the followingdescription is rather focused on the distinctive features or elements ofvarious embodiments of the present invention.

Embodiments of the present invention relate generally to semiconductordevices and more particularly, to a structure and method of forming aphotovoltaic cell. Specifically, embodiments of the present inventionmay relate to a solar cell which may be fabricated using a surfacetreatment process on an upper surface of an absorber layer to increasedevice performance.

Device performance may be described with parameters like open circuitvoltage, short circuit voltage, fill factor, and energy conversionefficiency. Open circuit voltage is a difference of electrical potentialenergy between two terminals of a device when no external electricalcurrent flows between the two terminals. Short circuit voltage is adifference of electrical potential energy between two terminals of adevice when external current is flowing between the two terminals. Afill factor is a ratio of actual maximum obtainable power to a productof open circuit voltage and short circuit voltage. Energy conversionefficiency is a ratio between electrical energy output and solar energyinput. A single crystal III-V semiconductor heterostructure with anintrinsic thin layer (HIT) cell may have a low open current voltage dueto surface Fermi level pinning. Strong surface Fermi level pinning maybe produced by a high surface state density. A method to reduce surfacestate density is needed to increase performance.

Embodiments of the present invention may improve open circuit voltage,fill factor, and energy conversion efficiency by performing a surfacetreatment on the upper surface of the absorber layer. The absorber layerof the HIT cell may be a single crystal III-V semiconductor. The surfacetreatment may include a hydrogen fluoride cleaning process, an ammoniumsulfide passivation process, and a plasma surface treatment. The surfacetreatment may improve device performance by permitting a more cohesiveinterface between the upper surface of the absorber layer and a lowersurface of a passivation layer. The more cohesive interface may allowcarriers to move from one layer to another with less resistance, andthus, increase device performance.

Methods of forming a photovoltaic cell and treating a surface of anabsorber layer to improve on-current voltage and fill factor aredescribed below with reference to FIGS. 1-9.

Referring now to FIG. 1, a cross section view illustrating a structure100 comprising the substrate 102 is shown. The substrate 102 may becomposed of a substance that yields a high open-circuit voltage and mayenable a substantially defect free junction to be formed between itselfand a back surface field layer formed in steps discussed below. In anembodiment, the substrate 102 may include any known bulk semiconductoror layered semiconductor such as Si/SiGe, a silicon-on-insulator (SOI),or a SiGe-on-insulator (SGOI). Examples of bulk semiconductor substratematerials may include undoped Si, n-doped Si, p-doped Si, single crystalSi, polycrystalline Si, amorphous Si, Ge, SiGe, SiC, SiGeC, Ga, GaAs,InAs, InP and all other III/V or II/VI compound semiconductors.Nonlimiting examples of III/V semiconductors include gallium arsenide,aluminum gallium arsenide, amorphous silicon, or any combinationthereof. In a preferred embodiment, the substrate 102 may be composed ofgallium arsenide. The substrate 102 may enable the solar cell to operateat higher voltages if composed of a wide bandgap III-V semiconductormaterial than if composed of a narrower bandgap material like amorphoussilicon. The substrate 102 may be formed using a conventional epitaxialdeposition process known in the art, such as, for example, rapid thermalchemical vapor deposition (RTCVD), low-energy plasma deposition (LEPD),ultra-high vacuum chemical vapor deposition (UHVCVD), atmosphericpressure chemical vapor deposition (APCVD), or molecular beam epitaxy(MBE). In an embodiment, the substrate 102 may be formed independently.In another embodiment, the substrate 102, the back surface field layer204, and the absorber layer 306 may be formed in a continuous epitaxialprocess.

Referring now to FIG. 2, a cross section view illustrating forming theback surface field layer 204 on an upper surface of the substrate 102 isshown. The back surface field layer 204 may reduce carrier recombinationat a back surface of a solar cell. In an embodiment, the back surfacefield layer 204 may reduce carrier recombination by repelling electronsfrom the back surface and attracting holes to the back surface. In anembodiment, the back surface field layer 204 may be composed of asemiconductor material, such as, for example, indium gallium phosphide.In an embodiment, the back surface field layer 204 may contain a p-typedoping agent, such as, for example, boron, aluminum, gallium, or acombination thereof. The back surface field layer 204 may be formedusing a conventional epitaxial deposition process known in the art, suchas, for example, RTCVD, LEPD, UHVCVD, APCVD, or MBE. The back surfacefield layer 204 may have a thickness ranging from approximately 35 nm toapproximately 65 nm.

Referring now to FIG. 3, a cross section view illustrating forming anabsorber layer 306 on the back surface layer 204 is shown. In anembodiment, the absorber layer 306 may be composed of a semiconductormaterial, such as, for example, gallium arsenide, aluminum galliumarsenide, amorphous silicon, or any combination thereof. In a preferredembodiment, the absorber layer 306 may be composed of gallium arsenide.The absorber layer 306 may contain a p-type doping agent, such as, forexample, boron, aluminum, gallium, or a combination thereof. Theabsorber layer 306 may have lighter doping than the back surface fieldlayer 204. The absorber layer 306 may be formed using a conventionalepitaxial deposition process known in the art, such as, for example,RTCVD, LEPD, UHVCVD, APCVD, or MBE. In an embodiment, the absorber layer306 may have a thickness ranging from approximately 2 micrometers toapproximately 4 micrometers, and ranges therebetween. The upper surfaceof the absorber layer 306 may have impurities, such as, for example,embedded oxides and elemental arsenic. The upper surface of the absorberlayer 306 may have dangling bonds which may result in a high surfacestate density. A high surface state density on the upper surface of theabsorber layer 306 may reduce solar cell performance. However, asdescribed below with reference to FIG. 4, a surface treatment may beperformed on an upper surface of the absorber layer 306 to improve solarcell performance.

In some embodiments, as described with reference to FIGS. 1-3, thesubstrate 102, the back surface field layer 204, and the absorber layer306 may be formed in a single deposition process. In other embodiments,as described with reference to FIGS. 1-3, the substrate 102, the backsurface field layer 204, and the absorber layer 306 may each be formedin separate deposition processes.

Referring now to FIG. 4, a cross section view illustrating an uppersurface 308 of the absorber layer 306 after a surface treatment isshown. In an embodiment, the surface treatment may include a hydrogenfluoride cleaning process, an ammonium sulfide passivation process, aplasma surface treatment, or any combination thereof. In a preferredembodiment, the hydrogen fluoride cleaning process may be performedbefore the ammonium sulfide passivation process. The hydrogen fluoridecleaning process may be performed on the upper surface 308 of theabsorber layer 306. The hydrogen fluoride cleaning process may removeimpurities, such as, for example, embedded oxides and elemental arsenic,from the upper surface 308 of the absorber layer 306. In an embodiment,an ammonium sulfide passivation process may be performed on the uppersurface 308 of the absorber layer 306. The ammonium sulfide passivationprocess may be used to remove impurities, such as, for example, embeddedoxides and elemental arsenic, from the upper surface 308 of the absorberlayer 306. The ammonium sulfide passivation process may create a sulfurmonolayer (not shown) on the upper surface 308. The sulfur monolayer onthe upper surface 308 may protect the upper surface 308 from obtainingadditional impurities. The sulfur monolayer on the upper surface 308 maybe substantially monatomic. The ammonium sulfide passivation process mayterminate dangling bonds at the upper surface 308 of the absorber layer306. By terminating dangling bonds, the surface state density on theupper surface 308 of the absorber layer 306 may be substantiallydecreased. A decrease in surface state density may reduce surface levelpinning. By reducing surface level pinning, surface recombinationvelocity may also decrease, resulting in increased open circuit voltage.A combination of hydrogen fluoride cleaning and ammonium sulfidepassivation may result in an improvement in open circuit voltage rangingfrom approximately a half bandgap to approximately a full bandgapvoltage.

In a preferred embodiment, a plasma surface treatment may be performedon the upper surface 308 of the absorber layer 306 after the hydrogensulfide cleaning process and the ammonium sulfide passivation process.The plasma surface treatment may be performed without exposing the uppersurface 308 to air. The plasma surface treatment may be performed byplasma enhanced chemical vapor deposition (PECVD). The plasma surfacetreatment may involve exposing the upper surface 308 to one or moreplasmas. The one or more plasmas may be transformed to a solid state onthe upper surface 308 and form a thin solid layer on the upper surface308. The solid thin layer may include material originating from the oneor more plasmas. The one or more plasmas exposed to the upper surface308 may include, for example, phosphine, hydrogen, nitrogen trifluoride,or any combination thereof. In an embodiment, the plasma surfacetreatment may involve exposing the upper surface 308 of the absorberlayer 306 to phosphine, hydrogen, and nitrogen trifluoride. The plasmasurface treatment involving phosphine, hydrogen, and nitrogentrifluoride may improve fill factor up to approximately 85%. Utilizingthe hydrogen fluoride cleaning process, the ammonium sulfide passivationprocess, and the plasma surface treatment may improve power conversionefficiency.

In an embodiment, the plasma surface treatment may be performed withoutthe hydrogen sulfide cleaning process or the ammonium sulfidepassivation process. In another embodiment, the plasma surface treatmentmay be performed before the ammonium sulfide passivation process. Inanother embodiment, the plasma surface treatment may be performed beforethe hydrogen sulfide cleaning process.

Referring now to FIG. 5, a cross section view illustrating forming apassivation layer 508 on the upper surface 308 of the absorber layer 306is shown. The passivation layer 508 may also be referred to as anintrinsic thin layer. The passivation layer (i.e. intrinsic thin layer)may be a significant feature in a HIT solar cell. The passivation layer508 may reduce carrier recombination at a front surface of a solar cellby reducing junction defects. In an embodiment, a reduction of junctiondefects may allow electrons to flow more freely out of the absorberlayer 306 and toward an front contact layer 814 (FIG. 8). In anembodiment, the passivation layer 508 may be composed of an amorphoussemiconductor material, such as, for example, amorphous silicon,amorphous germanium, or a combination thereof. In an embodiment, thepassivation layer 508 may be composed of an intrinsic semiconductor(i.e. undoped semiconductor), such as, for example, intrinsic silicon orintrinsic germanium. In a preferred embodiment, the passivation layer508 may be composed of an intrinsic amorphous semiconductor, such as,for example, intrinsic amorphous silicon or intrinsic amorphousgermanium. The passivation layer 508 may be formed using a conventionaldeposition technique, such as, for example, MBD, atomic layer deposition(ALD), chemical vapor deposition (CVD), physical vapor deposition (PVD),plasma enhanced CVD (PECVD), pulsed laser deposition (PLD), liquidsource misted chemical deposition (LSMCD), spin on deposition, orsputtering. In an embodiment, the passivation layer 508 may have athickness ranging from approximately 2 nm to approximately 8 nm.

Referring now to FIG. 6, a cross section view illustrating forming anemitter layer 610 on an upper surface of the passivation layer 508 isshown. In an embodiment, the emitter layer 610 may be composed of anamorphous semiconductor material, such as, for example, amorphoussilicon, amorphous germanium, or a combination thereof. The emitterlayer 610 may be doped to attract minority carriers and repel majoritycarriers. The emitter layer 610 may have an opposite-type dopant as theback surface field layer 204. For instance, if the back surface fieldlayer 204 contains a p-type dopant, the emitter layer 610 may contain ann-type dopant. Since the emitter layer 610 may have an opposite-typedopant as the back surface field layer 204, a photovoltaic effect may becreated. In an embodiment, the emitter layer 610 may contain an n-typedoping agent, such as, for example, nitrogen, phosphorus, arsenic,antimony, sulfur, or any combination thereof. The emitter layer 610 maybe formed using a conventional deposition technique, such as, forexample, MBD, ALD, CVD, PVD, PECVD, PLD, LSMCD, spin on deposition, orsputtering. Some conventional solar cell configurations may requiremetal organic chemical vapor deposition (MOCVD), such as, for example, aconventional single crystal p-type intrinsic layer n-type (PIN) solarcell. However, embodiments of the present invention may not requireMOCVD to form the emitter layer 610, resulting in a simpler fabricationprocess.

Referring now to FIG. 7, a cross section view illustrating forming atransparent electrode layer 712 on an upper surface of the emitter layer610 is shown. The transparent electrode layer 712 may be opticallytransparent and electrically conductive in a thin layer. The transparentelectrode layer 712 may include an oxide material, such as, for example,zinc oxide, indium oxide, or any combination thereof. In an embodiment,the transparent electrode layer 712 may be doped with a metal material,such as, for example, aluminum, tin, or a combination thereof. Forexample, the transparent electrode layer 712 may be composed of zincoxide and doped with aluminum. In another example, the transparentelectrode layer 712 may be composed of indium oxide and doped with tin.In another embodiment, the transparent electrode layer 712 may becomposed of a transparent conducting polymer, such as, for example,poly(3,4-ethylenedioxythiophene),poly(4,4-dioctylcyclopentadithiophene), or a combination thereof. Inanother embodiment, the transparent electrode layer 712 may be dopedwith poly(styrene sulfonate), iodine,2,3-dichloro-5,6-dicyano-1,4-benzoquinone, or a combination thereof. Thetransparent electrode layer 712 may be formed using any suitabledeposition technique known the art, including, for example, MBD, ALD,CVD, PVD, PECVD, PLD, LSMCD, spin on deposition, sputtering, orplatting.

Referring now to FIG. 8, a cross section view illustrating forming afront contact layer 814 on an upper surface of the transparent electrodelayer 712 is shown. The front contact layer 814 may be composed of ametal material, such as, for example, silver, copper, gold, aluminum, orany combination thereof. In a preferred embodiment, the front contactlayer 814 may be composed of silver. The front contact layer 814 may beformed using any deposition process known in the art, such as, forexample, screen printing, electroplating, electrophoretic deposition,underpotential deposition, chemical vapor deposition, physical vapordeposition, atomic layer deposition, or any combination thereof.

Referring now to FIG. 9, a cross section view illustrating forming aback contact layer 900 on a bottom surface of the substrate 102 isshown. The back contact layer 900 may serve as an electrical contact tothe bottom surface of the substrate 102. In an embodiment, the backcontact layer 900 may carry holes away from the substrate 102. The backcontact layer 900 may be composed of a conductive metal, such as, forexample, silver, copper, gold, aluminum, titanium gold, titaniumpalladium gold, or any combination thereof. In a preferred embodiment,the back contact layer 900 may be composed of titanium gold. The backcontact layer 900 may be formed using any deposition process known inthe art, such as, for example, electroplating, electrophoreticdeposition, underpotential deposition, CVD, low pressure CVD, liquidsourced misted chemical deposition (LSMCD), PVD, ALD, pulsed laserdeposition (PLD), sputtering, or any combination thereof.

Embodiments of the present invention may provide a structure and methodof forming a HIT solar cell. The HIT solar cell may involve simplerfabrication processes than conventional solar cell configurations andprovide substantially the same performance as conventional solar cellconfigurations. A conventional single crystal PIN solar cell may requireMOCVD to form an emitter layer on an intrinsic passivation layer,however, embodiments of the present invention may not require MOCVD toform the emitter layer 610 on the passivation layer 508, resulting insimpler fabrication than conventional methods. Although embodiments ofthe present invention may involve a simpler fabrication process, theymay have a lower open circuit voltage and a lower fuel factor thanconventional solar cell configurations without a surface treatment.However, open circuit voltage and fuel factor may be improved byimplementing the surface treatment discussed in reference to FIG. 4.Specifically, a combination of hydrogen fluoride cleaning and ammoniumsulfide passivation may result in an improvement in the solar cell'sopen circuit voltage ranging from approximately a half bandgap toapproximately a full bandgap voltage. In addition, the plasma surfacetreatment involving phosphine plasma, hydrogen plasma, and nitrogentrifluoride plasma may improve fill factor up to approximately 85%.Utilizing the hydrogen fluoride cleaning process, ammonium sulfidepassivation, and the plasma surface treatment may improve powerconversion efficiency to substantially the same level as a conventionalsingle crystal PIN solar cell.

The descriptions of the various embodiments of the present inventionhave been presented for purposes of illustration, but are not intendedto be exhaustive or limited to the embodiments disclosed. Manymodifications and variations will be apparent to those of ordinary skillin the art without departing from the scope and spirit of the describedembodiments. The terminology used herein was chosen to best explain theprinciples of the embodiment, the practical application or technicalimprovement over technologies found in the marketplace, or to enableothers of ordinary skill in the art to understand the embodimentsdisclosed herein.

What is claimed is:
 1. A heterostructure with an intrinsic thin layer(HIT) solar cell structure comprising: a back surface layer on an uppersurface of a substrate; an absorber layer on an upper surface of theback surface layer; a sulfur monolayer on an upper surface of theabsorber layer; a thin solid layer on an upper surface of the absorberlayer; a passivation layer on the upper surface of the absorber layer;an emitter layer on the upper surface of the passivation layer; atransparent electrode layer on an upper surface of the emitter layer; afront contact layer on an upper surface of the transparent electrodelayer; and a back contact layer on a bottom surface of the substrate. 2.The structure of claim 1, wherein the sulfur monolayer terminatesdangling bonds on the surface of the absorber layer, resulting inincreased open circuit voltage ranging from a half bandgap to a fullbandgap voltage.
 3. The structure of claim 1, wherein the thin solidlayer comprises phosphine, hydrogen, nitrogen trifluoride, or anycombination thereof.
 4. The structure of claim 1, wherein the thin solidlayer comprises phosphine, hydrogen, and nitrogen trifluoride.